Abstract
The taxonomic status, distribution, abundance and reproduction of shags in New Zealand are described. Evidence is presented to justify a change in the present acceptance of Stictocarbo as a monospecific genus containing three races. The genus should be separated into two monotypic species, the Spotted Shag S. punctatus and the Pitt Island Shag S. featherstoni. The race S.p. steadi (Blue Shag) should no longer be recognised as distinct from the Spotted Shag. The dimorphic Stewart Island Shag Leucocarbo carunculatus chalconotus is split into two allopatric populations which differ in the proportions of the plumage morphs and in body dimensions of individuals.
The diets of shags were deduced from diagnostic prey remains in regurgitated pellets, stomach contents and nestling vomits. Equations were constructed to define relationships between the original size of prey items and the size of their diagnostic remains: otoliths in fish, exoskeletal remains in crustaceans and beaks in cephalopods. Otoliths of the common teleost fish in Otago waters are presented in an atlas of scanning electron micrographs.
The compositions of the marine diets of five shag species in southern New Zealand are presented in terms of numbers, weights and size ranges of prey taken. Fish predominate in all diets. The most important prey items recorded from the birds were: Black Shag Phalacrocorax carbo: yellow-eyed mullet Aldrichetta forsteri (from Otago) and freshwater eels Anguilla (from Chatham Island). Little Shag P. melanoleucos (from Otago): cockabullies (Trypterygiidae). Stewart Island Shag (Otago) and Chatham Island Shag L.c. onslowi (from Chatham Island): flatfish (Pleuronectidae), cockabullies and opalfish Hemerocoetes. Spotted Shag (Otago): ahuru Auchenoceros punctatus. Pitt Island Shag (Chatham Island): cockabullies.
Quantitative aspects of diving behaviour were recorded from seven shag species in New Zealand and Australia and comparative information was collected for nine other species of diving birds from five families. Pause times increased linearly with increases in dive times for all but one species and the relationships between these two parameters exhibited interspecific differences on a phylogenetic basis. Diving efficiency (Dive+pause) in shags decreased through the genera Phalacrocorax, Leucocarbo and Stictocarbo. Relationships between dive time and water depth were species-specific and fell into one of two patterns. Demersal feeders exhibited linear increases in dive time with increasing water depth and dive times at all depths were proportional to the size of the bird. In contrast, pelagic feeders exhibited relatively constant dive times independent of water depth.
A combination of morphometric analyses can define the habits of shags to a degree sufficient to account for the ecological separation between species. An indication of the main foraging style employed, either pelagic or demersal, can be deduced from body shape and some idea of the type and size of prey can be deduced from bill structure. For most demersal feeders, maximum dive time in minutes and 10x maximum diving depth in metres approximately equal body weight in kilograms. The type of nesting site, either "tree" or "ground", and flight performance can both be deduced from wing loadings.